java中单例模式

public class QuotationEngine {
private static QuotationEngine instance;// 对象实例

private QuotationEngine() {
}

public static QuotationEngine getInstance() {
if (null == instance) {
synchronized (QuotationEngine.class) {
if (null == instance) {
instance = new QuotationEngine();
}
}
}

return instance;
}
}